Am I correct that they lower ph? How often and how much would you have to use to keep the ph constant? Thanks
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If you read the entire blog it has some tips on trying different amounts and testing.
They can soften water, which can lower pH and/or make it easier to lower pH and keep it low using chemicals. There's no good answer to your question because it really depends on the hardness of the water you're starting with and what is making it hard.
I started out using oak leaves and actually got some of the Tannin items because they looked like good spawning sites for licorice gouramis. I did not do a post-boil soak because I wanted the tannins that get leached out in that soak to end up in my tank. This is the blackest my water has ever been, though I haven't bothered with a pH test.
Last year when I had Dicrossus and chocolate gourami spawns, each tank had a pH of between 2-4 using oak leaves. It's worth noting, though, that I was also using rain water to fill them. I only bother to test pH when I have spawns, so I can't tell you much about how things impact it.
Adding color like this to the water does make it a little harder to TEST pH reliably, of course...